1. Field of the Invention
The present invention relates to a valve timing control apparatus in an internal combustion engine, and more particularly to a valve timing control apparatus for controlling the timing of at least either one of the opening timing and the closing timing of the intake valve and the exhaust valve.
2. Description of the Art
The valve timing control apparatus (hereafter referred to simply as the valve control apparatus) is used to vary, for example, the start timing or the end timing of an intake or exhaust according to the operating condition of the engine to thereby improve the intake efficiency and the exhaust efficiency in the cylinder. To this end, the valve control apparatus performs a control operation so as to change the rotational phase of the camshaft with respect to the crankshaft in order to advance (advance control) or retard (retard control) the timing of at least one of the intake valve and the exhaust valve driven by the cam rotors on the camshaft.
A conventional valve control apparatus is shown in JP-A-61-268810. This apparatus has mounted between the cam pulley rotating synchronously with the crankshaft and the camshaft an intermediate gear which engages both the cam pulley and the camshaft in helical spline engagement. This intermediate gear is arranged so as to be slidable in the direction of the camshaft by a hydraulic pressure and a spring pressure. The engaging portion of the intermediate gear serves to impart a torque to the camshaft, so that the camshaft is rotated in the rotating direction and the relative position of the cam pulley changes with respect to the camshaft.
With respect to a hydraulically-operated valve control apparatus, its hydraulic control method is disclosed in JP-A-1-305112. This method estimates the condition of the hydraulic oil from the working fluid temperature in the hydraulic system, the cooling water temperature and the number of engine revolutions and corrects the hydraulic control value according to the detected condition of the working fluid. By this method, even if the performance of the hydraulic system changes due to a viscosity change of the working fluid and the occurrence of bubbles in the working fluid, the valve control apparatus can control the valves as specified.
However, in the above-mentioned valve control apparatus, if a hydraulic system is used, for example, since the hydraulic system is driven by the motive power of the engine, its performance varies with the number of engine revolutions. Therefore, even if the opening and closing timing of the valve is decided according to the operating condition of the engine, the valve control apparatus suffers errors in the stage of valve operation control and does not have a satisfactory controllability,
Conventionally, when performing control to advance or retard the valve opening and closing timing, there has been a difference between the controllability of advance control and the controllability of the retard control. This results from the fact that in valve timing control, to change the rotational phase of the camshaft relative to the crankshaft, for example, a working torque is applied to the camshaft, but the working torque required differs between in advance control and in retard control even when the camshaft is rotated for the same rotational phase. As a result, if the same manner of control is performed in advancing and retarding valve timing, the controllability of either advancing or retarding valve timing or both deteriorates, so that satisfactory valve timing control can not be achieved.